My friend’s kid used to take ~1 shit a week. What happened is, he got constipated one time and had a very painful shit. Naturally, he didn’t like it, who would, but was too young to understand the situation was abnormal. So, logically, in order to avoid pain while shitty he decided to avoid shitting as long as possible, which resulted in painful shits because it’s a week’s worth coming out at once.

Ended up being self-fulfilling. This went on for at least several months if not longer than a year.

So, to answer how not to shit for 3 days; apparently willpower is enough.